(1) Field of the Invention
This invention generally relates to a remote control system in which pulse sequences which serve as commands are transmitted in a wireless mode, after modulation on an appropriate carrier signal, to one or a plurality of receivers. The transmitter usually forms part of a hand-held unit controllable by a user while the receiver forms part of the apparatus to be controlled, for example an audio set, a video set or as occurs to an ever increasing extent at present, a hearing aid.
The invention particularly relates to such a receiver.
In the following description the remote control system will be abbreviated to RC-systems, the transmitter used therein to RC-transmitter and the receiver to RC-receiver.
(2) Description of the Prior Art
In a remote control system the RC-transmitter includes a message generator which in response to a relevant request by the user supplies a message which consists of a plurality of clock pulse-modulated information pulses. For the transmission of this message to the RC-receiver, an infrared modulation circuit is nowadays usually provided, which includes an infrared LED whose radiated infrared light is modulated by the modulated pulses in the pulse train.
The RC-receiver then includes an infrared photodiode which converts the received infrared light into an electric signal. This signal is applied to a signal processing circuit for generating the original message.
Embodiments of said RC-transmitters and RC-receivers are extensively described in the reference. The RC-receivers described there comprise a cascade arrangement of an input circuit, a modulation circuit, a pulse shaper and an output circuit. The input circuit receives the output signal of the photodiode and includes an input resonant circuit which is tuned to a frequency which corresponds to the clock pulse frequency (38 kHz here). In addition, this input circuit includes an amplifier circuit in the form of a differential amplifier which amplifies the output signal of the input resonant circuit and applies it to the demodulation circuit. This input circuit acts as a selective amplifier circuit which prevents the proper operation of the RC-receiver from being disturbed by other types of light sources which also transmit infrared light, such as the sun, incandescent lamps etc. So as to still further suppress the disadvantageous influence of these light sources, the demodulator circuit is provided with a demodulation resonant circuit which is also tuned to said clock pulse frequency.
Of this prior art infrared RC-receivers the amplifier circuit, the demodulation circuit, the pulse shaper, the output circuit and a still further number of circuits are jointly designed as one integrated circuit. However, in order to obtain the required selectivity, both the input resonant circuit and the demodulation resonant circuit are designed as discrete components, such as capacitors and, more specifically inductances. As a result thereof, the complete RC-receiver is too big to be accommodated in very small housings, such as, for example, in hearing aids. This problem can be obviated to a very high extent by the use of gyrators by means of which these discrete components can be translated to integratable circuitry. This would indeed solve the problem of bulk to a very great extent, but it has an adverse effect on the ever increasing demand for integrated circuits operating at a low voltage in the order of magnitude of 1.2 V. By way of comparison it should be noted that the RC-receiver described in the reference operates at an approximately 5 V supply voltage. For those RC-receivers this is however no problem since they will usually form part of an apparatus which derives its supply voltage from the mains supply.